Unlike fossil fuels such as coal and petroleum (hydrocarbons), hydrogen is a clean fuel that burns to form water without producing greenhouse gases, such as carbon dioxide. Fossil fuels on the earth, which have been naturally used in the past, are getting exhausted today. For this reason, there is an urgent need to establish a method for producing hydrogen as a fuel to be replaced with fossil fuels. Reflecting this background, various research and development activities have been carried out all over the world as is well known, and some excellent methods have already been proposed on the level of laboratory experiments (e.g., a method described in Patent Literature 1 mentioned below, which utilizes catalytic pyrolysis of hydrocarbons using a metal catalyst, etc.). However, for the production of hydrogen on an industrial scale, it is necessary to solve problems of (1) how to produce hydrogen at low cost as a matter of course, (2) how to store it compactly and (3) how to transport it safely. The problems (2) and (3) are particularly difficult to solve, because hydrogen is the smallest and lightest molecule with weak intermolecular interactions. Solutions possible at present include a method to apply high pressure, a method to maintain hydrogen at very-low temperature below the boiling point of hydrogen (−253° C.), a method to use a hydrogen storage alloy, and a method to charge hydrogen in a metal tank at high pressure (e.g., 350 atm to 1000 atm), etc. However none of these methods provides any fundamental solution.
In light of the above-mentioned points, in an attempt to provide a novel method for producing hydrogen as a solution to problems to be solved for the production of hydrogen on an industrial scale, such as problems of production cost, storability and transportability, the present inventors have proposed in Patent Literature 2 a method using an aqueous solution of formic acid as a feedstock, in which formic acid is subjected to hydrothermolysis at a temperature from 250° C. to 600° C. (hydrothermal reaction: reaction in a sealed reaction vessel in which high-temperature and high-pressure water is involved), whereby formic acid is decarboxylated (decarboxylation: HCOOH→H2+CO2) to produce hydrogen.    Patent Literature 1: JP-A-2003-95605    Patent Literature 2: JP-A-2005-289742